Many machines and off-road vehicles include unsynchronized mechanical transmissions, often in addition to a hydraulic transmission such as a hydrostatic continuously variable transmission (CVT). Machines and vehicles with such a hydro-mechanical transmission have the benefits of infinite gear ratios and high torque when using the hydrostatic transmission at low vehicle speeds, while at higher speeds the mechanical transmission is generally more efficient.
There are a number of reasons for using an unsynchronized mechanical transmission. For example, synchronizers are expensive and can wear out too easily. However, the omission of synchronizers can lead to difficulties when shifting gear, for example when the machine or vehicle is stationary and a large load is present on the mechanical gearing. In such case, the transmission selecting member may not be able to easily disengage the existing gear when attempting to shift to a neutral gear position. Furthermore, when attempting to shift from the neutral gear position to select one of the gears, the transmission selecting member and the gear may not be properly aligned such that shifting to that gear is not possible.
The conventional solution for such transmission arrangements is for the operator to slowly drive the machine or vehicle in the relevant direction using the hydrostatic CVT until the load on the mechanical gearing is removed, thus allowing the desired gear to be selected. However, this requires skill from the operator and it is possible for the transmission to become stuck between gears. Also, such an approach can cause damage to the transmission.